class Bird:
    def fly(self):
        print('小鸟正在飞')


class Plane:
    def fly(self):
        print('飞机正在飞')


class XtDog:
    def fly(self):
        print('哮天犬正在飞')


def fly(obj):
    obj.fly()

fly(Plane())
fly(Bird())
fly(XtDog())
